Mark,
The PST information came from a researched Rosemay Waring, Ph.D. I believe she is out of the UK. I don’t know of any standard labs or alternative labs that do that specific test. What many people have done is look at Urinary Sulfates. Quest Diagnostic offers this test as an insurance based lab – http://www.questdiagnostics.com/testcenter/TestDetail.action?ntc=11013. What can also be done is a plasma amino acid test too. With autism, what has been seen commonly is low serum sulfate and high urine sulfate.